Passing negative electrical current into the interior of a nerve cell in the cortex produces hyperpolarization and inhibition of the cortical neuron. Inhibition is followed by post-inhibitory depolarization and discharges. Conversely, passing positive electrical current into the interior of a cortical neuron produces depolarization and excitation of the neuron followed by post-excitatory hyperpolarization. High frequency depolarizing pulses applied to a neuron does not produce after-discharges but a prolonged post-excitatory hyperpolarization of the cell membrane.